Recently, single-stranded multimode and single-mode optical fibers have seen increased application in communications and many other related systems. Consequently, the fabrication of optical fiber circuit elements, such as access couplers for interconnecting separated terminals, has gained in importance. Several successful methods for producing low-loss access couplers using multimode single-stranded optical fibers have been developed. Two rather simple procedures relied upon in the fabrication of low-loss access couplers (excess loss of the order of 1 dB) for multimode fibers were recorded by Barnoski and Friedrich in the Applied Optics 15, 2629 (1976) and by Ozeki and Kawasaki in Applied Physics Letters 28, 528 (1976).
The first method employs a CW CO.sub.2 laser to fuse two sections of multimode fibers side-by-side. The power coupling transmission coefficient was 6.6%. The second method uses two biconically tapered multimode fibers to form a coupler by cementing these fibers side-by-side with an optical adhesive. The two biconical sections are produced by pulling a multimode fiber while it is being heated by a circular tungsten wire (such a technique is disclosed in the article appearing in Applied Physics Letter 26,386 (1975) by T. Ozeki, T. Ito and T. Tamura). While the second method reported values of 62 and 79% for throughput and 4.6 and 9.6% for power coupling transmission coefficients, improved very low-loss access couplers (less than 1 dB) and coupling ratios up to 24% were reported by Kawasaki and Hill in Applied Optics 16, 1794 (1977) using a combination of the two methods referred to above. This produced a tapered coulper fabricated by fusing the two twisted fibers under stress with the aid of an oxy-butane microtorch. A partial explanation as to the reason why the twisted, tapered biconical couplers provide efficient performance is given in the Ozeki-Kawasaki Applied Physics Letter article identified above.
Further improvements of the multiport directional couplers have evolved using Kawasaki and Hill's latest technique. E. G. Rawson and A. B. Nafarrate further demonstrated this technique and their article appearing in the Electronics Letters 14, 274 (1978) discloses that efficient multiport star couplers were obtained by twisting several fibers under tension and fusing them with the aid of a microtorch.
Unfortunately the couplers, while being satisfactory to one degree or another when made in the laboratory under controlled personally supervised individual operations, did not lend themselves to a relatively predictable fabrication sequence which could be duplicated for mass production of a multitude of such couplers. Thus, there is a continuing need in the state-of-the-art for a method for producing fiber optic couplers which is cost efficient and which provides controllable, predictable, acceptable coupling ratios between at least two fibers while assuring their structural integrity.